1. Field of the Invention
The present invention relates to a spark plug for an internal combustion engine and, more particularly, to a small-sized spark plug with a fitting piece on which an M10 screw portion is mounted.
2. Description of the Prior Art
Recently, for improving output of an internal combustion engine, it is necessary to make large a valve size of an intake manifold or an exhaust manifold. Thus, in order to keep a space surrounding the combustion chamber of the engine, it is required for miniaturizing a spark plug fitted to the internal combustion engine, that is, making small a diameter of a fitting piece of the spark plug.
As conventional small-sized spark plugs, for example, there is disclosed a spark plug in U.S. Pat. No. 6,147,441.
However, in the conventional small-sized spark plug for the internal combustion engine, which has an M10 (ISO2704) screw portion fitted to the fitting piece thereof, especially, a wall thickness of the fitting piece is small so that it is also required to miniature a ground electrode welded to the fitting piece, such as, to make thin the ground electrode.
Therefore, it is impossible to secure the heat resistance of the ground electrode itself due to the miniaturization of the ground electrode, so that there is the possibility that it is unable to sufficiently project a spark position into the combustion chamber of the engine.
Furthermore, because the screw portion has a small-size with a screw diameter of substantially M10, a distance between an insulator supporting a center electrode and the fitting piece is made short, so that side sparks, that is, electric discharges which are generated from the center electrode to the insulator are easily caused, whereby it is hard to generate normal discharges between the center electrode and the ground electrode.
The present invention is directed to overcome the foregoing problems. Accordingly, it is an object of the present invention to provided a spark plug for an internal combustion engine, which is configured to permit a spark position to sufficiently project, while preventing side sparks from generating, so as to make sure an ignitability thereof.
In order to achieve such object, according to one aspect of the present invention, there is provided a spark plug for an internal combustion engine, comprising a center electrode with an end portion; an insulator surrounding a periphery of the center electrode to support the center electrode; a fitting piece that supports the insulator, the fitting piece being provided with an end surface and at a periphery thereof with a fitting screw portion, the end portion of the center electrode projecting from the end surface of the fitting piece, the fitting screw portion having a diameter of M10; a noble metal chip with one end surface and other end surface opposite thereto, the one end surface being mounted on the end portion of the center electrode, the other end surface being arranged so that an axial distance A between the end surface of the fitting piece and the other end surface of the noble metal chip is set within a range of 3 mm to 8 mm, the other end surface having an area S which is set within a range of 0.07 mm2 to 0.55 mm2; and a ground electrode having one end portion and other end portion, the one end portion thereof being fitted to the end surface of the fitting piece, the other end portion being inclined toward the noble metal chip at a slant angle of xcex8 with respect to an axial length of the spark plug so as to provide a discharge gap G between the noble chip and the other end portion, the slant angle xcex8 being set within a range of 40xc2x0 to 70xc2x0, the discharge gap G being set within a range of 0.7 mm to 0.9 mm.
According to the structure of the present invention, in the spark plug for an internal combustion engine having the fitting piece with the M10 screw portion, because the projection length A, the slant angle xcex8, the area S of the other end surface of the noble metal chip and the discharge gap G are configured to have the above dimensions, respectively, it is possible to sufficiently protrude the spark position of the spark plug, and to prevent side sparks from occurring.
That is, in a case where the projection length (dimension) A between the end surface of the fitting piece and the other end surface of the noble metal chip is less than 3 mm, when the side sparks occur, creepage distances in the surface of the insulator between the center electrode and the fitting piece are made short, the side sparks easily occur.
Moreover, the projection length A is less than 3 mm, it is unable to sufficiently protrude the center electrode in a combustion chamber of the internal combustion engine so that it is impossible to secure a sufficient ignitability. On the contrary, in a case where the projection length A is more than 8 mm , it must be necessary not only to make long the ground electrode corresponding to the projection length A, but also to arrange the tip end portion of the ground electrode in an center side of the combustion chamber so that it must be required for using a ground electrode which has a large heat resistance more than needs.
As described above, the projection length A between the end surface of the fitting piece and the other end surface of the noble metal chip is preferably set within a range defined by the expression:
3 mmxe2x89xa6Axe2x89xa68 mm.
In addition, in a case where the slant angle xcex8 is less than 40xc2x0, the configuration of the exhaustion portion of the noble metal chip which is opposite to the ground electrode is too be inclined so that in order to secure a sufficient exhaustion performance of the noble metal chip, it must be required for the noble metal chip having enough length.
However, the longer the length of the noble metal chip is, it is more hard not only to fit the noble metal chip to the one end portion of the center electrode and but also to increase the required amount of the noble metal chip, thereby increasing the cost of the spark plug.
On the contrary, in a case where the slant angle xcex8 is more than 70xc2x0, it must be necessary to make long the ground electrode for providing the discharge gap between the other end surface of the noble metal chip and the ground electrode so that the heat resistance of the ground electrode is deteriorated, thereby causing an oxidation of the ground electrode.
As the reasons described above, the slant angle xcex8 is preferably set within a range defined by the expression:
40xc2x0xe2x89xa6xcex8xe2x89xa670xc2x0.
In addition, in a case where the area S of the other end surface of the noble metal chip is less than 0.07 mm2, because the exhaustion amount of the noble metal chip strongly increases, it is not practical to use the noble metal chip.
On the contrary, in a case where the area S of the other end surface of the noble metal chip is more than 0.55 mm2, in the process of the growth of a flame core which is generated by an electric discharge occurs in the discharge gap, because the area of the other end surface of the noble metal chip is large, the flame core is easy to contact to the ground electrode so that the contact of the flame core to the ground electrode causes the flame extinction phenomenon such as the transfer of the flame core, thereby deteriorating the ignitability of the spark plug.
As the reasons described above, the area S of the other end surface of the noble metal chip is preferably set within a range defined by the expression:
0.07 mm2xe2x89xa6Sxe2x89xa60.55 mm2.
Incidentally, according to the structure of the spark plug with the noble metal chip whose other end surface has the area S within the range of 0.07 mm2 to 0.55 mm2, the discharge gap G is preferably set within a range defined by the expression:
0.7 mmxe2x89xa6Gxe2x89xa60.9 mm.
However, in a case where the ground side noble metal chip is mounted on the ground electrode and opposite to the noble metal chip mounted on the end portion of the center electrode, the discharge gap G is preferably set within a range defined by the expression:
0.5 mmxe2x89xa6Gxe2x89xa60.9 mm.
The reason is that, because the ground side metal chip is mounted on the ground electrode, as compared with the spark plug without having the ground side noble metal chip, in the process of the growth of the flame core which is generated in the discharge gap, it is possible to prevent the flame core from contacting to the ground electrode 6, thereby improving the ignitability of the spark plug. However, in a case where the discharge gap G is less than 0.5 mm, even if the ground side noble metal chip is mounted on the ground electrode, it is impossible to obtain a sufficient ignitability.
In addition, the area S2 of the end surface of the ground side noble metal chip is preferably set within a range defined by the expression:
0.12 mm2xe2x89xa6S2xe2x89xa60.80 mm2.
In a case where the area S2 of the end surface of the ground side noble metal chip is less than 0.12 mm2, it is insufficient to exhaust the ground side noble metal chip, thereby to deteriorate the durability of the spark plug. In addition, in a case where the area S2 is more than 0.80 mm2, the ignitability of the spark plug is deteriorated.
Furthermore, the height h between the end surface thereof and the surface of the other end portion of the ground electrode is preferably set within a range defined by the expression:
0.3 mmxe2x89xa6hxe2x89xa61.5 mm.
In a case where the height h between the end surface thereof and the surface of the other end portion of the ground electrode is less than 0.3 mm, it is insufficient to use an edge effect of the ground side noble metal chip, thereby deteriorating the ignitability of the spark plug. In addition, in a case where the height h is more than 1.5 mm, because it is insufficient to transfer the heat in the ground side noble metal chip to the ground electrode, the ground side noble metal chip gets to have high temperature, and the high temperature of the ground side noble metal chip causes the noble side metal chip to be largely exhausted.
Furthermore, it is preferable that the noble metal chip is made of at least one of Ir (iridium) alloy whose at least 60 wt % consists of Ir, such as pure Ir, Irxe2x80x94Pt (platin) alloy, Irxe2x80x94Rh alloy and Irxe2x80x94Ni (nickel) alloy, and Pt alloy at least 60 wt % of which consists of Pt and is formed by adding at least one of pure Pt, Ir, Ni, Pd (palladium), Ru (ruthenium) and W (tungsten).
By using the noble metal chip made by the above materials, it is possible to provide the spark plug for an internal combustion engine, which has a superior durability.
Still furthermore, it is possible that the insulator has a substantially cylindrical shape and is formed at a chip side end with a body portion whole of which has a same diameter in an axial direction thereof.
According to the structure, because the body portion is formed to the chip side end portion of the insulator, it is possible to extend a least distance between the insulator and the fitting piece in an air, making it possible to prevent the generation of the side spark from the center electrode to the ground electrode.